Zn and Cu isotopic variability in the Alexandrinka volcanic-hosted massive sulphide (VHMS) ore deposit, Urals, Russia

Copper and Zn isotope ratios of well-characterized samples from three ore facies in the Devonian Alexandrinka volcanic-hosted massive sulphide (VHMS) deposit, southern Urals, were measured using multi collector ICP-MS (MC-ICP-MS) and show variations linked to depositional environment and mineralogy. The samples analysed derived from: a) hydrothermal-metasomatic vein stockwork, b) a hydrothermal vent chimney, and c) reworked elastic sulphides. As the deposit has not been significantly deformed or metamorphosed after its formation, it represents a pristine example of ancient seafloor mineralization. Variations in delta Cu-65 (where delta Cu-65=[(Cu-65/Cu-63)(sample)(Cu-65/Cu-63)(standard)-1]*1000) and delta Zn-66 (where delta Zn-66 = [(Zn-66/Zn-64)(sample)/(Zn-66/Zn-64)(standard)-1]*1000) of 0.63 and 0.66 parts per thousand, respectively, are significantly greater than analytical uncertainty for both isotope ratios (+/- 0.07 parts per thousand, 2 sigma). Very limited isotopic fractionation is observed in primary Cu minerals from the stockwork and chimney, whereas the Zn isotopic composition of the stockwork varies significantly with the mineralogy. Chalcopyrite-bearing samples from the stock-work have lighter delta Zn-66 by -0.4 parts per thousand relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. delta Zn-66 also showed significant variation in the chimney, with an enrichment in heavy isotopes toward the chimney rim of similar to 0.26 parts per thousand, which may be caused by changing temperature (hence fractionation factor), or Raleigh distillation. Post-depositional seafloor oxidative dissolution and re-precipitation in the clastic sediments, possibly coupled with leaching, led to systematic negative shifts in Cu and Zn isotope compositions relative to the primary sulphides. Copper shows the most pronounced fractionation, consistent with the reduction of Cu(II) to Cu(I) during supergene mineralization. However, the restricted range in delta Cu-65 is unlike modem sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3 parts per thousand has been observed [Rouxel, O., Fouquet, Y, Ludden, J.N., 2004. Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev sea-floor hydrothermal fields on the Mid-Atlantic Ridge. Econ. Geol. 99, 585-600; Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S., Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma source mass spectrometry: implications for use as geochemical tracers. Chem. Geol. 163, 139-149]. (c) 2005 Elsevier B.V. All rights reserved.